Add a bit to a virtual frame element telling if it's been copied. Set...

Add a bit to a virtual frame element telling if it's been copied.  Set
it when a copy is made, clear it when the element is next written.

Committed: http://code.google.com/p/v8/source/detail?r=1529

[Erik Corry, 2009-03-18 10:31:02]

LGTM despite the comment.

http://codereview.chromium.org/42324/diff/1/2
File src/virtual-frame.h (right):

http://codereview.chromium.org/42324/diff/1/2#newcode107
Line 107: type_ = (type_ & ~IsCopiedField::mask()) |
IsCopiedField::encode(true);
The BitField class feels a bit heavyweight.  This line just does:

type_ |= kIsCopiedFieldMask;

[William Hesse, 2009-03-18 10:34:54]

LGTM